Saltwater intrusion into freshwater habitats may significantly alter ecosystem-level dynamics. Saltwater intrusion derives from human interventions (damming rivers; freshwater diversion; etc.) and natural variability (droughts or increases in sea level). Increased salinity leads to a cascade of events that impact coastal ecosystems at a variety of scales ranging from small (cm to m) scale changes in sediment biogeochemistry and materials processing to large (m to km) scale alterations in the distribution of flora and fauna. Despite the global scope of this impact, many of the effects of saltwater intrusion on coastal ecology and biogeochemistry remain poorly understood or undocumented.
The key impacts of saltwater intrusion likely stem from the increases in chloride (salt) and sulfate concentration. Small changes in chloride concentration can physiologically stress microorganisms, plants, and animals and alter metabolic pathways, rates of activity, and abundance. Sulfate is a terminal electron acceptor for anaerobic respiration that is present in limited amounts in freshwater but that is abundant in seawater. Changes in sulfate availability may result in dramatic changes in sediment biogeochemistry, which may drive ecosystem-level changes in plant and animal distributions.
We are examining the response of coastal freshwater marshes to changes in salinity and sulfate levels. The centerpiece of our work is an effort to quantify the interplay between geochemical factors and microbial community composition and activity in freshwater, brackish, and marine sediments. Field and laboratory geochemical, microbiological, molecular biological, and ecological data help unravel the network of biogeochemical reactions governing the cycles of carbon, oxygen, sulfur, nutrients, and trace metals in freshwater, brackish, and marine sediments.
Coastal areas are the most populated and impacted regions on Earth. Forty-six percent of the US population resides within coastal watersheds, and a larger fraction impacts these environments intermittently via recreational and vocational activities. An obvious impact of coastal development is eutrophication, but a more subtle impact is saltwater intrusion into freshwater habitats. Saltwater intrusion results from natural processes (drought, sea level rise, or changes in freshwater delivery), and anthropogenic causes (damming rivers; freshwater removal for municipal, industrial, or agricultural use; changes in freshwater inflow). Increased salinity can influence coastal ecosystems at small (the distribution of microbially-mediated processes) and large (the distribution of plants) scales. Despite the global scope of saltwater intrusion, and concern by management agencies, its effects on coastal ecology and biogeochemistry remain poorly understood.
Saltwater intrusion into freshwater habitats increases the ionic strength of pore fluids but also increases sulfate (SO42-) availability. Differences in biogeochemical redox zonation between freshwater and saltwater sediments are attributed to differences in sulfate availability: in sulfate-limited freshwater sediments, methanogenesis and iron reduction dominates terminal organic carbon metabolism, while in sulfate-rich marine sediments, sulfate reduction dominates terminal organic carbon metabolism. Based solely on energetics, sulfate reduction should precede methanogenesis when both groups of microbes compete for the same substrates; however, field data from coastal sediments has documented contemporaneous methanogenesis and sulfate reduction. Co-existence of metabolic processes and contemporaneous use of two or more terminal electron acceptors (TEAs) within the same redox zone arises from the use of non-competitive substrates, the activity of facultative microbes, or spatial heterogeneity (i.e., the presence of distinct chemical microzones within a given redox horizon).
Increased sulfate availability may drive cascades of effects that impact the pathways and rates of elemental transformation and the recycling efficiency of other elements, including carbon, nitrogen, phosphorus, sulfur, and iron. By increasing recycling of organic carbon, sulfate may reduce organic matter preservation in soils, and hence sediment accretion rates and the ability of marshes to keep pace with rising sea levels. Sulfides—the product of sulfate reduction—are toxic to both plants and animals, and increased sulfate availability may thus constrain the distributions of plants and animals that lack the ability to adapt to high sulfide concentrations. At present we have little understanding of how much the effects of seawater intrusion are driven by salinity (changes in ionic strength) versus increased sulfate availability. Disentangling these two effects is critical for an improved mechanistic understanding of seawater intrusion and ecosystem change along salinity gradients.
We conducted a laboratory experiment utilizing flow through bioreactors to evaluate the time scale upon which geochemical and microbial dynamics were influenced by moderate changes in salinity (Weston et al. (2006) JGR Biogeosciences 111:(G1):G01009, 10.1029/2005JG00007). A 10‰ increase in salinity resulted in rapid and dramatic changes in microbial activity, materials fluxes, and organic carbon mineralization rates. Ammonium release from sediments increased rapidly in response to increased salinity; most of the ammonium was desorbed at low salinity (10‰).
After a week of increased salinity, rates of organic carbon mineralization were significantly higher at 10% salinity relative to freshwater controls; sulfate reduction rapidly replaced methanogenesis as the dominant metabolic mode of sediment microorganisms in the salinity-amended treatments.
Increased salinity resulted in a number of significant geochemical changes. In salinity amended sediments, ammonium, phosphate, and silicate fluxes increased by 20 to 38%, reduced iron fluxes increased by ~150%, methane fluxes decreased by 77%, and in situ organic carbon mineralization rates increased by ~110%.
Edmonds, J. E., N. B Weston, S. B. Joye, X. Mou, and M. A. Moran, 2009. Microbial Community Response to Seawater Amendment in Low-Salinity Tidal Sediments. Microbial Ecology, 58: 558-568.
Craft, C., J. Clough, J. Ehman, H. Guo, S. B. Joye, M. Machmuller, D. Park, and S. Pennings, 2009. Effects of accelerated sea level rise on ecosystem services provided by tidal marshes: A simulation of the Georgia Coast. Frontiers in Ecology and Environment, 7(2): 73-78.
Edmonds, J. W., N. B. Weston, S. B. Joye and M. A. Moran, 2008. Variation in prokaryotic community composition as a function of resource availability in tidal creek sediments. Applied and Environmental Microbiology, 74: 1836-1844.
Weston, N. B., R. Dixon, and S. B. Joye, 2006. Microbial and geochemical ramifications of salinity intrusion into tidal freshwater sediments. Journal of Geophysical Research: Biogeosciences, 111: (G1): G01009, 10.1029/2005JG00007.
Environmental Protection Agency
NSF Division of Environmental Biology